1. Field of the Invention
The invention relates to separations conducted in the gas or vapour phase. In some embodiments, the separation is conducted simultaneously with waste heat recovery, refrigeration or heat pumping. The invention may be applied to air separation and numerous other gas separation or purification processes.
This application is a continuation-in-part of my copending U.S. patent application Ser. No. 06/866,395 filed 5-23-86, now Pat. No. 4,702,903, which is a continuation-in-part of Ser. No. 06/538,370 filed 10-3-83, abandoned.
2. Prior Art
Gas separation by pressure swing adsorption is achieved by cyclically reversing flow of a gas mixture over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorbent bed, and is reduced during alternating intervals of flow in the reverse direction. According to the well known parametric pumping principle, the less readily adsorbed component tends to migrate in the first direction over complete cycles, while the more readily adsorbed component tends to migrate in the reverse direction, thus achieving separation.
In the idealized parametric pumping concept, a more readily sorbed component of a fluid mixture is cyclically immobilized and liberated on a fixed bed by preferential sorption and desorption caused by cyclic modulation of an external parameter, which in general may be pressure, temperature, pH or other adjustable parameter. When the parameter is adjusted to maximize sorption loading of the more readily sorbed component on the bed, the fluid contacting the bed is made to flow in a first direction through the bed. During alternating intervals when the parameter is adjusted to minimize sorption loading of the more readily sorbed component on the bed, the fluid is made to flow in the reverse direction along the same flow path in the bed. Over complete cycles, the less readily sorbed fraction of the mixture tends to migrate in the first direction, while the more readily sorbed component tends to migrate in the reverse direction, thus achieving separation. In the ideal parametric pumping concept, there is essentially no flow along the flow path in the bed except when sorption of the more readily component is maximized or minimized. An example of the parametric pumping for liquid phase separation using temperature as the parameter was described by Wilhelm et al (R.H. Wilhelm, A.W. Rice and A.R. Bendelius, Ind. Eng. Chem. Fundamentals 5, 141, (1966)). When the parameter is pressure and the fluid mixture is compressible as in pressure swing adsorption, there must be flow in the fixed adsorbent bed while the pressure is changing, causing large departures from an ideal parametric pumping process. Because flow in the adsorbent bed cannot be suppressed at intermediate pressures between the maximum and minimum limits, prior art pressure swing adsorption processes for gas separation have failed to approach ideal separation performance and efficiency.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds with directional valving to control the flow of compressed feed gas over each bed in alternating sequence, while the other bed is purged at low pressure by the reverse flow of a portion of the product gas, which is the less readily adsorbed component. While this less readily adsorbed fraction can be highly purified, the more readily adsorbed fraction cannot be totally purified because of mixing with the less readily adsorbed component in the purge gas, and recovery of the less readily adsorbed product is incomplete. This conventional process makes inefficient use of mechanical energy, because the compression energy of the feed gas is largely dissipated during expansion processes. Another common name for the pressure swing adsorption separation process is "heatless adsorption", which seems to deny the possibility of beneficial effects by thermal coupling to a regenerative thermodynamic cycle to improve pressure swing adsorption apparatus as disclosed in the present invention.
Some secondary and adverse thermal effects do arise in operation of conventional pressure swing adsorption gas separation apparatus, particularly those using large adsorption beds with poor heat exchange to ambient. The adverse effects include cyclic release and take-up of the latent heat of adsorption, causing a temperature swing of the adsorbent bed acting in opposition to the pressure swing, and in larger beds also leading ot detrimental radial temperature gradients.
As mentioned above, the usual pressure swing adsorption cycle has the performance limitations that the more readily adsorbed component cannot be purified completely (because of mixing with the purge), and therefore the less readily adsorbed component of the feed mixture cannot be recovered completely. Hence, a conventional pressure swing adsorption system used to recover hydrogen from the purge stream of an ammonia plant could deliver highly pure hydrogen, but cannot recover all the hydrogen.
A conventional pressure swing adsorption plant applied to air separation cannot deliver oxygen with purity greater than about 95%, because argon is concentrated with oxygen in the less readily adsorbed fraction over zeolite molecular sieves, on which nitrogen is the more readily adsorbed component based on equilibrium selectivity. An alternative air separation cycle based on kinetic selectivity over carbon molecular sieves or tight pore zeolites can deliver highly pure inert gas since nitrogen and argon form the less readily adsorbed component, but can only achieve a limited enrichment of oxygen which is the more readily adsorbed component in this case. There is a need for an improved pressure swing adsorption process which can deliver oxygen of at least 99% purity.
The more general object of adapting pressure swing adsorption to achieve substantially complete fractionation of a binary mixture has been addressed in U.S. Pat. No. 3,149,934 (Martin) and U.S. Pat. No. 4,354,859 (Keller et al). In these inventions, the feed mixture is injected between the ends of each adsorbent bed, the pressure and flow regime is coordinated to concentrate the more readily adsorbed component to one end and the less readily adsorbed component to the other end of the bed, and the more readily adsorbed component is refluxed into its end of the bed during the high pressure phase of the cycle while the less readily adsorbed component is refluxed into the other end during the low pressure end of the cycle. With reflux at both ends of the bed, it becomes possible in principle to produce both components with high purity and recovery. In the conventional cycle, only the less readily adsorbed component is refluxed during purge, resulting in the inability to purify both components of the binary mixture. It may be noted that an inverted cycle has been disclosd in U.S. Pat. No. 4,359,328 (Wilson), which has a high pressure reflux of the more readily adsorbed fraction (but no low pressure purge, improving purification of the more readily adsorbed fraction but losing the full ability to purify the less readily adsorbed fraction.
The Keller patent is also of interest because it achieves the coordination of total pressure and flows in the adsorbent bed through the use of pistons or other mechanical volume displacement means at both ends of the bed. The pistons are specified to have unequal displacements. The cyclic flow and pressure regime is generated by reciprocating the pistons at a suitable periodic frequency, and with a specified range of phase angles between them such that a two component mixture can be separated to a high extent.
None of the above cited references contemplates the direct coupling of a pressure swing adsorption separation process to a regenerating thermodynamic cycle as in the present invention. The prior art does not anticipate use of a variable geometry adsorbent bed with cyclically varied volume to compensate the compressibility effects which have prevented close approach of pressure swing adsorption processes to the parametric pumping ideal.